A LED light source is known from WO2010137002A1, and is shown together with a phase cut dimmer of the trailing edge type in FIG. 1.
In FIG. 1, K1 and K2 are input terminals for connection to a supply voltage source supplying an AC supply voltage such as the mains supply. Bi-directional switch S1, snubber capacitor C1 and timing circuitry TC are comprised in a phase cut dimmer of the trailing edge type. Terminals K3 and K4 are input terminals of a rectifier formed by diodes D5-D8. Terminal K4 is connected to input terminal K2. Input terminal K1 is connected to terminal K3 via bidirectional switch S1.
A first output terminal and a second output terminal of the rectifier are connected by a first series arrangement of a resistor R-WB and switch S2 and also by a second series arrangement of a resistor R-SB and switch S3. A control electrode of switch S2 is coupled to an output of comparator COMP1 and a control electrode of switch S3 is coupled to an output terminal of comparator COMP2. A first input terminal of comparator COMP1 and a first input terminal of comparator COMP2 are both connected to the first output terminal of the rectifier. At a second input terminal of comparator COMP1 a reference voltage Vref1 is present and at a second input terminal of comparator COMP2 a reference voltage Vref2 is present. Resistor R-WB, switch S2 and comparator COMP1 together form a first bleeder and resistor R-SB, switch S3 and comparator COMP2 together form a second bleeder.
The first and second output terminals of the rectifier are also connected by means of a series arrangement comprising a diode D9 and a capacitor C2.
The function of the first bleeder is to charge the snubber capacitor C1, when the bidirectional switch S1 has become non-conductive and the diode D9 is blocking. The function of the second bleeder is to charge the power supply of the dimmer (not shown) and reset the timing circuitry comprised in the dimmer.
Respective input terminals of converter CONV are connected to respective sides of capacitor C2. Output terminals of the converter CONV are connected to a LED load LED. Converter CONV is a converter for generating a current through the LED load LED out of a voltage present across capacitor C2.
The first and second output terminals of the rectifier are also connected by a series arrangement of resistors R2 and R3. Resistor R3 is shunted by capacitor C3. Resistors R2 and R3 together with capacitor C3 form a low pass filter for generating a dim signal. The input terminals of the low pass filter are formed by the first and second output terminals of the rectifier and, during operation, the dim signal is present across the capacitor C3. A common terminal of resistor R2 and capacitor C3 is connected to a dim input terminal of converter CONV, so that the dim signal is supplied to this dim input terminal of the converter.
When the mains supply is connected to input terminals K1 and K2, the sinusoidal supply voltage supplied by the mains supply is phase cut by the phase cut dimmer and the phase cut sinusoidal supply voltage is rectified by means of the rectifier.
Since the phase cut dimmer is of the trailing edge kind, during each half period of the AC supply voltage the dimmer switch is first maintained conductive and then switched off at the adjusted phase angle of the phase cut dimmer. The dimmer switch is subsequently maintained in a non-conductive state until the end of the half period.
The rectified phase cut mains supply voltage (when its momentary magnitude is higher than the voltage across the capacitor C2) causes a charging current to flow via diode D9 to capacitor C2. The voltage across the capacitor is used to supply the converter CONV and thus also to supply the LED load LED connected to its output terminals. The converter generates a current through the LED loads.
To ensure a proper operation of the phase cut dimmer, in spite of the fact that the LED light source consumes less current than an incandescent lamp for which the phase cut dimmer was actually designed, the known LED light source is further equipped with the first bleeder and the second bleeder connected between the first and second output terminals of the rectifier. The first bleeder carries a comparatively small current and is switched on when the voltage between the first and second rectifier output terminals is below a first predetermined value (e.g. 200V). The second bleeder carries a higher current and is only switched on when the voltage between the first and second rectifier output terminals drops below a second predetermined value (e.g. 50 V), much lower than the first predetermined value.
The magnitude of the current through the LED load depends on the dim signal that is supplied to the dim input of the converter and thus is a function of the shape of the voltage present between the output terminals of the rectifier and hence also a function of the adjusted phase angle of the phase cut dimmer.
In case the phase cut dimmer were used to dim an incandescent lamp, the dim signal present at the output terminals of the low pass filter would have a different value for each value of the adjusted phase angle of the phase angle dimmer. This is because the voltage between the first and second output terminals of the rectifier would have a steep edge at the adjusted phase angle of the phase angle dimmer (or in other words at the moment the dimmer switch S1 is rendered non-conductive). In the case of the LED light source described hereinabove, however, some undesired effects occur when the phase cut dimmer is of the trailing edge type and the adjusted phase angle is between 90 degrees and the value of the phase angle for which the voltage between the first and second rectifier output terminals equals the first predetermined value (in other words the voltage at which the first bleeder is activated). In this case, the capacitor C2 supplying the converter is charged to a voltage that equals the amplitude of the mains supply voltage, when the phase angle is 90 degrees. When the phase angle increases further, no current flows to the capacitor, since its voltage is higher than the momentary amplitude of the voltage present between the first output terminal and the second output terminal of the rectifier. Flow of current in the opposite direction is impeded by the diode. Since the impedance of the snubber capacitor C1 comprised in the dimmer is much lower than the input impedance of the LED light source, the voltage between the first and second output terminals of the rectifier follows the shape of the mains voltage until the first bleeder is activated, irrespective of whether the dimmer switch is conductive or not. As a consequence, the dim signal present between the output terminals of the low pass filter is the same for all adjusted phase angle values that lie between 90 degrees and the phase angle at which the first bleeder is activated. This causes a discontinuity in the dimming curve, i.e. the relation between adjusted phase angle and light output of the LED load LED.
Furthermore, when the dimmer switch is made non-conductive after the first bleeder has been activated, the bleeder current causes a slight distortion of the dim signal, resulting in non-linearity of the relation between the adjusted phase angle and the light output of the LED load. Since the phase angle is adjusted by a user, it is desirable that the relation between adjusted phase angle of the phase cut dimmer and the light output is free of discontinuities and non-linearities.
In FIG. 2, several voltages in the circuitry shown in FIG. 1 are shown as a function of time. In FIG. 2, switch S1 is made non-conductive 6 msec. after the zero crossing of the mains voltage and the first bleeder is activated after 8 msec. Curve V1 is the mains supply voltage, curve V2 is the voltage across the capacitor C2, V3 is the voltage between the first and second output terminals of the rectifier and curve V4 is the shape of the voltage across the output terminals of the rectifier in case diode D9, capacitor C2, converter CONV and LED load LED were jointly replaced by an incandescent lamp. In curve V4 there is a steep trailing edge when the switch S1 in the phase cut dimmer is made non-conductive. This trailing edge is far steeper than curve V3, because the incandescent lamp draws more current than the LED light source, and hence capacitor C1 is charged quickly.
It can be seen that curve V3 has exactly the same shape as the mains voltage between 5 msec (a phase angle of 90 degrees) and 8 msec, whereas after 8 msec curve V3 is dropping somewhat steeper than the mains supply voltage, but not as steep as curve V4, because the time constant of the RC circuit R_WB*C1 is larger than it would be if the load were an incandescent lamp.